1. Field of the Invention
The invention relates to a personal computer using a flash memory as its BIOS-ROM.
The invention also relates to a personal computer suitable for externally rewriting the contents of its BIOS-ROM.
2. Description of the Related Art
In general, a computer system, such as a personal computer, is equipped with a ROM (read only memory) for storing a BIOS (basic input/output system). Heretofore, in the event of destruction of the contents of the BIOS-ROM or revision of the BIOS, the BIOS-ROM has had to be replaced by a new chip. In order to replace the ROM chip with a new one, it is required to open the computer casing. The work of opening the computer casing and replacing the ROM chip with a new one is very troublesome.
Flash memories have been put on the market recently as rewritable ROMs. They have various features. One of them is that stored data can be erased block by block. Thus, the use of a flash memory as the BIOS-ROM would be convenient.
The flash memory is generally equipped with a control read-only area, called a boot block, at the end of its storage area. In the case where this type of flash memory is used as the BIOS-ROM, therefore, it is required to make access to the boot block in order to execute a far jump instruction first immediately after the CPU has been reset. In the subsequent normal state, it is required to make access to another area storing the BIOS. It is preferable that the BIOS be compatible with existing personal computers.
The boot block in which a far jump instruction is placed overlaps an area for storing the BIOS compatible with existing personal computers on CPU-visible memory space (address space). In order to make access to the boot block to execute the far jump instruction immediately after the power-on reset and access to the BIOS storage area in the subsequent normal state, some consideration will be needed for controlling address data.
In addition, some consideration will be needed to allow areas within the BIOS-ROM that are not accessed in the normal state, i.e., memory space allocated to other areas, such as the boot block, than the area storing the BIOS, to be opened to other than the BIOS-ROM access.